CONTROL STRATEGY FOR MARINE RISER OF POSITION-MOORED VESSEL IN OPEN AND LEVEL ICE-COVERED SEA NGUYEN HOANG DAT NATIONAL UNIVERSITY OF SINGAPORE 2011 CONTROL STRATEGY FOR MARINE RISER OF POSITION-MOORED VESSEL IN OPEN AND LEVEL ICE-COVERED SEA NGUYEN HOANG DAT (B.Eng. (Hons.), HCMUT) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS First of all, I would like to express my sincere appreciation to my supervisor, Professor Quek Ser Tong, at the Department of Civil Engineering, National University of Singapore (NUS), for his encouraging and providing continuous guidance during my research. He was always willing to answer my questions, check my results and suggest new problems. I am also grateful to Professor Asgeir J. Sørensen, at the Center for Ships and Ocean Structures at the Norwegian University of Science and Technology (NTNU), for his support which permitted my experimental studies at NTNU. A special thank to my co-supervisor, Dr. Nguyen Trong Dong, at Marine Cybernetics (Trondheim, Norway), for his advices and encouragements to this research. He always shares his knowledge regarding marine control systems and this helps me a lot in my research. I also express my appreciation to NTNU laboratory staff, Torgeir Wahl and Knut Arne Hegstad, for helping with the experimental set-up. I would like to thank Dr. John Halkyard from John Halkyard & Associates for answering my questions concerning riser and mooring systems. Finally, I would like to acknowledge my parents and my wife who have demonstrated through example the meaning of commitment and understanding. Their loves are always my strong support during the whole difficult time of my research. This work has been supported by the NUS Research Scholarship. The experiments presented in this thesis have been carried out at the Center for Ship and Ocean Structures (CeSOS), NTNU. The Research Council of Norway is the main sponsor of CeSOS. i TABLE OF CONTENTS Acknowledgements .i Table of Contents ii Summary .vi List of Tables .viii List of Figures .ix List of Symbols xiii List of Abbreviations xvii Chapter 1. Introduction .18 1.1 Background and Motivation .18 1.2 Brief Review of Previous Works 20 1.2.1 Modelling of Marine Risers .20 1.2.2 Control of Marine Risers .25 1.2.3 Control of Riser End Angles 26 1.2.4 Station Keeping for Drilling Operations in Ice-covered Sea .27 1.3 Objectives and Scope .30 1.4 Outline of Thesis 31 Chapter 2. Model of Vessel-Mooring-Riser System 33 2.1 Introduction .33 2.2 Kinematics and Coordinate Systems .33 2.3 Model of Riser .36 2.3.1 Governing Equation of Motion 36 2.3.2 Stiffness Model .38 2.3.3 Inertia Model 41 2.3.4 Damping Model 42 2.3.5 Load Model .43 ii 2.3.6 Governing Equation of Motion in FE method 44 2.4 Multi-cable Mooring System 46 2.4.1 Force in a Mooring Line 47 2.4.2 Restoring Force from Spread Mooring System 52 2.4.3 Total Contributions from Mooring System 53 2.5 Model of Vessel Motion .54 2.5.1 Low Frequency Vessel Model .55 2.5.2 Linear Wave-frequency Vessel Model .61 2.6 Concluding Remarks 62 Chapter 3. Control of Riser End Angles by Position Mooring .63 3.1 Introduction .63 3.2 Measurement of Top and Bottom Riser Angles 63 3.3 Structure of Control System .64 3.4 Control Plant Model of Vessel and Riser 65 3.4.1 Control Plant Model of Vessel 65 3.4.2 Control Plant Model of Riser Angles .68 3.5 Plant Control of Vessel-riser-mooring System 69 3.5.1 Nonlinear Passive Observer .69 3.5.2 Control of Mooring Line Tension 71 3.5.3 Mooring Line Allocation .74 3.5.4 Heading Control by Thrusters .78 3.6 Local Optimization: Optimal Set-point Chasing .78 3.6.1 Optimal Vessel Position accounting for Riser Angle Criterion .78 3.6.2 Reference Model .79 3.7 Numerical Simulations .80 3.7.1 Problem Definition 80 3.7.2 Effect of Vessel Offset on REAs .82 3.7.3 Effect of Position Mooring Control .83 3.7.4 Comparison with DP System .86 3.8 Experimental Tests .87 3.8.1 Experimental Set-up 88 3.8.2 Experimental Results .90 iii 3.9 Conclusions .92 Chapter 4. Minimization of Riser Bending Stresses 94 4.1 Introduction .94 4.2 Calculation of Riser Bending Stresses 94 4.3 Control Criterion based on End Angles 95 4.3.1 Problem Statements .95 4.3.2 Simulation Results and Discussions 96 4.4 Control Criterion based on End Bending Stresses . 100 4.4.1 Optimal Set-point Chasing 100 4.4.2 Simulation Results and Discussions 101 4.5 Conclusions . 104 Chapter 5. Control of Position Mooring Systems in Ice-covered Sea . 106 5.1 Introduction . 106 5.2 Level Ice Load Model 107 5.2.1 Determination of Contact 111 5.2.2 Crushing and Bending Failure . 112 5.3 Vessel-ice Interaction Model 115 5.4 Numerical Example 118 5.5 Simulation Results . 121 5.5.1 Effect of Vessel Offset on REAs . 121 5.5.2 Effect of Position Mooring Control . 122 5.6 Conclusions . 128 Chapter 6. Conclusions and Future Works 129 6.1 Summary of Key Points . 129 6.2 Conclusions . 130 6.3 Recommendations for Further Work 131 References 133 Appendix A. Marine Cybernetics Laboratory (MCLab) and Cybership III Model . 142 A.1 Marine Cybernetics Laboratory (MCLab) 142 A.2 Cybership III Model . 145 iv Appendix B. Marine Systems Simulator . 149 Appendix C. Publications and Submitted Papers . 150 C.1 Journal Papers 150 C.2 Conference Papers . 150 v SUMMARY Increasing safety and efficiency of drilling operation is a challenging research topic in offshore engineering especially when the operating location changes. Ensuring that marine risers remain functional during operations under “normal” environmental condition is critical. The main objective of the study is to present a control strategy for maintaining small end angles of marine riser during shallow water drilling operations by position-moored (PM) systems in open water and level ice-covered sea. Basically, an active positioning control using mooring line tensioning to reduce the riser end angles (REAs) in open sea is first formulated and illustrated numerically. Model experimental tests are then performed to validate the proposed control strategy. In addition, stresses along the riser due to bending are considered numerically, including the case where end bending stiffeners are used, which requires the REA control criterion to be replaced by one with terms related to the stresses at the riser ends. The proposed REA control strategy using line tensioning with vessel set-point chasing algorithm is extended for operation in level ice-covered sea. In the normal drilling and work-over operations, the riser angles at the well-head and top joint must be kept within an allowable limit (ideally within ±2o) to prevent the drilling string wearing against the ball joints and guarantee continuous drilling operation. Although this can be achieved by applying sufficient tension at the top of the riser, this may lead to higher stresses, requiring the use of pipes with higher strength or dimensions. Alternatively, the vessel may be moved to reduce the mean offset, and hence the REAs, by tensioning the mooring lines under normal environmental conditions. This minimizes the need for and/or fuel consumption of thrusters to control the surge and sway. In this study, the minimization of vessel offset and REAs of the vessel-mooringriser system is achieved by automatically changing the lengths of the mooring lines based on optimal set-point chasing. To design the control strategy, the mathematical model of the riser, mooring system and vessel is formulated. The riser is modeled using beam elements which include the flexural stiffness, since the latter can contribute significantly in shallow water condition. The cable catenary equation is used to analyze the mooring line and the effect of mooring is applied on the hull as position-dependent vi external forces. The effect of the motions of the hull subjected to the environmental loads is integrated into the system by imposing externally defined oscillations at the top end of the riser. The effectiveness of the strategy is demonstrated by numerical simulations and experiments of a moored vessel. The simulations are conducted using the Marine System Simulator (MSS) developed by the Norwegian University of Science and Technology (NTNU) with some modifications to integrate the multi-cable mooring system and riser finite element model. The experiments were performed in the Marine Cybernetics Laboratory (MCLab) at the NTNU using the Cybership model vessel, which is a 1:120 scaled model of the vessel used in the numerical simulation. Bending stress in the riser may be a controlling factor in the performance of marine operations and hence studied herein. It is observed that for riser with hingeconnected ends, executing the proposed PM control reduces its bending stresses considerably. Hence its material/geometry can be optimally proportioned such that both the allowable limits of the REAs and the stresses are not exceeded. For the case where the riser is fitted with end bending stiffeners, the control criterion can be modified to account for the end bending stresses instead of REAs. The control strategy is shown to be similarly effective numerically. The Arctic region is one of the most difficult areas to work in due to its remoteness, the extreme cold, and presence of dangerous sea ice. Normal dynamic positioning (DP) systems may not operate satisfactorily in ice-covered sea since they are designed for open water. For moored system in ice, it seems that no active control of PM system has been implemented with respect to riser performance. Therefore the control strategy for PM system proposed herein is extended to level ice-covered sea. For simulating ice-vessel interactions, an ice-breaking process is adopted, which considers the coupling between the vessel motion and the ice-breaking process. To validate the control performance in ice-covered sea, the vessel is first exposed to open water and then to level ice regime with different ice thicknesses. Numerically simulated results support the implementation of the proposed vessel set-point chasing algorithm using REA criterion in conjunction with line tensioning for moored vessel operating in level ice. vii LIST OF TABLES Table 3.1. Vessel main parameters .81 Table 3.2. Properties of mooring lines .81 Table 3.3. Properties of riser 82 Table 3.4. Force of thrusters (normalized using values obtained by Case 3) .87 Table 5.1. Drill ship’s main parameters 119 Table 5.2. Properties of riser 119 Table 5.3. Ice parameters . 120 viii References [24] Fard M. P. and Sagatun S. I. (2001). 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The laboratory is used for testing sea keeping, fixed and floating offshore structures, and mooring systems. It is also suitable for more specialised hydrodynamic tests, mainly due to the advanced towing carriage, which has capability for precise movement of models in degrees of freedom. Position camera Marker Towing carriage Figure A.1. MCLab at NTNU 142 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model Figure A.2. Close-up view of position camera Riser angle monitor Position monitor Wave generator PC Host PC Figure A.3. Control room in MCLab The DHI wave maker at one end of the water tank (Figure A.4) can generate regular and irregular waves with maximum wave height Hs = 0.3 m and period T = 0.6 – 1.5 s. The position monitoring is performed by a Qualisys monitoring system with three cameras mounted on the towing carriage (Figures A.1 and A.2). The cameras 143 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model monitor the position of markers mounted on the vessel (Figure A.1) and transmit data to a computer in the control room (Figure A.3). This computer is installed with Qualisys Track Manager (QTM) software. This software computes the position of the center of gravity (CG) of Cybership III, based on the data from the cameras and predefined distances from the markers to the CG. The calculated position in DOFs is then transmitted to the onboard computer of the vessel. When implementing experimental tests, there is a wireless communication between the onboard computer and the computer in the control room. Positions and control signal are then constantly transmitted to the control room, and users can adjust the control parameters which are transmitted back to the onboard computer. The control interface is loaded into the Opal-RT environment, which governs the communication between the vessel and the control room. The software used in the MCLab was developed using rapid prototyping techniques and automatic code generation under Matlab/Simulink® and Opal-RT. The target computer onboard the vessel runs the QNX real-time operating system while experimental results are presented in real-time on a host computer in the control room. Figure A.4. Wave generator 144 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model A.2 Cybership III Model The Cybership III (see Figure A.5) using in the MCLab has been developed in cooperation between the Department of Marine Technology and the Department of Engineering Cybernetics at NTNU for testing dynamic position system and navigational system. Details of the project were described in Nilsen (2003). Before the Cybership III, the Cybership I and II have been frequently used, but they both have limitations to their usage due to model size, control method and propulsion system. The Cybership I is a 1:70 scaled model of a thruster controlled supply vessel for DP, having a mass of m = 17.6 kg, length L = 1.19 m and equipped with controlled azimuth thrusters with independent controllable azimuth angles. The Cybership II is a 1:70 scaled model of a multipurpose supply vessel for DP and tracking control, having a mass of m = 15 kg, length L = 1.15 m and equipped with aft azimuth thrusters with rudders, fore azimuth thruster and tunnel thruster at the bow. Anchor Sail winch Figure A.5. Cybership III The Cybership III combined the best parts of its predecessors by using a propulsion system similar to that on the Cybership I and the computer system identical to that on the Cybership II. It is a 1:30 scaled model of a supply vessel, having a mass 145 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model m = 75 kg, length L = 2.27 m and breath B = 0.4 m. Mechanical and electric configuration/installation of the Cybership III were developed by Nilsen (2003). The vessel is equipped with two main aft azimuth thrusters, one fore tunnel thruster and one fore azimuth thruster (Figure A.6). The power system includes four 12V-18Ah batteries which can sustain the ship systems for at least one day of experiments without charging. The internal hardware architecture is controlled by an onboard PC which can communicate with an onshore PC through a WLAN. The PC onboard the ship (target PC) uses the QNX real-time operating system. The control system is developed on a PC in the control room (host PC) under Simulink/Opal and downloaded to the target PC using an automatic C-code generation and a wireless Ethernet. Figure A.6. Two aft azimuth thrusters (left), fore azimuth thruster and fore tunnel thruster (right) of Cybership III The motion capture unit (MCU) manufactured by Qualysis provides the Earthfixed position and heading of the vessel. The MCU consists of three onshore cameras mounted on the towing carriage. The cameras emit infrared light and receive the light reflected form the markers on the ship. In order to test the PM control strategy a turret mooring system was designed and mounted on the Cybership III, as shown in Figure A.7. The mooring system 146 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model consists of four mooring lines; each attached at one end to the turret via sail winches, and at the other to an anchor mounted on the tank wall. Strain gauges were attached to the lines to measure the tensions during the experiment. The HITEC HS-785HB sail winches (Figure A.7) installed on the turret of the vessel were used to obtain the required lengths by pulling or releasing the lines. These sail winches are derived forms of heavy duty servo with many advanced features that provide good performance and reliability. Each winch can rotate forward or backward up to 2.5 turns, depending on the input voltage. Each turn will wind about 120 mm of the line. The drilling riser was modelled by a plastic pipe with outer diameter of mm. To provide higher mass, lead was put inside the riser pipe. The riser was subjected to tension at the top, which was modelled by a tensioned spring. The riser was installed just below the turret. In order to measure the REAs, two reflected light markers were attached at the top and bottom of the riser. An underwater camera (see Figure A.8) at the bottom of the water tank was used to capture these marker motions. From these signals, the REAs can be computed. Sail winch Mooring line Rotatable turret Strain gauge Riser Figure A.7. Mooring turret mounted on Cybership III 147 Appendix A Marine cybernetics laboratory (MCLab) and Cybership III model Figure A.8. Underwater camera 148 Appendix B Marine systems simulator APPENDIX B. MARINE SYSTEMS SIMULATOR The Marine Systems Simulator (MSS), which was developed and introduced by Fossen and Perez (2004) at NTNU, is a Matlab/Simulink library and simulator for marine systems (Figure B.1). It includes models for ships, underwater vehicles, and floating structures. The library also contains guidance, navigation, and control (GNC) blocks for real-time simulation. An overview of the MSS has been presented by Perez et al. (2005). There are three main toolboxes in the MSS: Marine GNC (Guidance and Navigation Control) toolbox, MCSim (Marine Cybernetics Simulator) and DCMV (Dynamics and Control of Marine Vehicles). The Marine GNC toolbox was firstly developed by Fossen T. I. and his students at NTNU as a supporting tool for his book (Fossen, 2002). The MCSim was a complete simulator of DP marine operations, which was developed by Sørensen A. J. and Smogeli ∅. N. at NTNU (Sørensen et al., 2003). The DCMV toolbox was developed by Perez and Blanke (2003) for autopilot design. In this study, the multi-cable mooring system, riser FEM model and level ice model were added in the original MSS to execute the proposed PM control strategy. Figure B.1. Marine systems simulator 149 Appendix C Publications and submitted papers APPENDIX C. PUBLICATIONS AND SUBMITTED PAPERS C.1 Journal Papers [1] Nguyen H. D., Nguyen T. D., Quek S. T. and Sørensen A. J. (2010). Control of marine riser end angles by position mooring. Journal of Control Engineering Practice, Vol. 18, No. 9, pp. 1013-1021. [2] Nguyen H. D., Nguyen T. D., Quek S. T. and Sørensen A. J. (2011). Positionmoored drilling vessel in level ice by control of riser end angles. Journal of Cold Regions Science and Technology, Vol. 66, No. 2-3, pp. 65-74. C.2 Conference Papers [3] Nguyen H. D., Quek S. T. and Nguyen T. D. (2007). Modelling of drilling marine riser: Comparisons in static analysis. In Proceedings of 1st International Conference on Modern Design, Construction and Maintenance of Structures (MDCMS), Vol. 2, pp. 243–252, Hanoi, Vietnam, December 10–11. [4] Nguyen H. D. and Quek S. T. (2007). Comparison of cable and tensioned beam models for marine riser. In Proceedings of Twentieth KKCNN Symposium on Civil Engineering, Jeju, Korea, October. [5] Nguyen H. D., Nguyen T. D., Quek S. T. and Sørensen A. J. (2008). Control of marine drilling riser angles by position mooring. In Proceedings of the Marine Operations Specialty Symposium 2008 – MOSS-2008, pp. 367–379, CORE, National University of Singapore, Singapore, March 5–7. [6] Nguyen H. D., Nguyen T. D., Quek S. T. and Sørensen A. J. (2009). Design of marine riser angle control by position mooring. In Proceedings of TwentySecond KKCNN Symposium on Civil Engineering, Chiengmai, Thailand, October 31 – November 2. [7] Nguyen H. D., Nguyen T. D. and Quek S. T. (2010). Control of marine riser in level-ice environment. In Proceedings of Twenty-Third KKCNN Symposium on Civil Engineering, Taipei, Taiwan, November 13 – 15. 150 [...]... modelling of marine risers will be presented Next, the review continues with a brief review of operational control of marine risers Subsequently, the review will focus on the dynamic positioning (DP) and position mooring (PM) for minimizations of the REAs in open water Final, a review on station keeping for drilling operations in ice- covered sea is summarized 1.2.1 Modelling of Marine Risers Marine riser. .. development of time domain techniques, which allows the nonlinearities, for riser applications 1.2.2 Control of Marine Risers Active control of vibrating slender structures have been investigated and implemented in many industrial applications In offshore engineering, these structures include marine risers, free hanging underwater pipelines, and drill strings for oil and gas operations Fard and Sagatun... between vessel hull and ice ν Vessel velocity vector νr Relative velocity vector between vessel and current ρa Density of air ρf Density of internal fluid of riser ρw Density of sea water σ Bending stress of riser σb Bottom end bending stress of riser σc Crushing strength of ice σe von Mises equivalent stress of riser pipe σf Bending strength of ice σpr Radial stress of riser pipe σpz Axial stress of riser. .. stress of riser pipe σt Top end bending stress of riser σy Yield strength of steel τcmoor Control force from mooring line τcr Vector of ice crushing load ice Ice load τmoor Mooring force vector in body-fixed frame τthr Thrust force vector * vessel xv τwave2 Second-order wave load vector τwind Wind load vector υ Poisson coefficient φves Stem angle of vessel hull ϕe Angle of axial axis of riser element in. .. global coordinate ice Opening angle of ice wedge ψves Slope angle of vessel hull xvi LIST OF ABBREVIATIONS API American Petroleum Institute CG Centre of gravity DCMV Dynamics and control of marine vehicles DNV Det Norske Veritas DOF Degree of freedom DP Dynamic positioning FE Finite element FEM Finite element method FPSO Floating Production Storage and Offloading GNC Guidance and navigation control JONSWAP... stiffness matrix of riser element L Length of vessel Lc Crushing depth Lm Length of mooring line lc Characteristic length of ice M System inertia matrix of vessel Mr Total mass matrix of entire riser ma Hydrodynamic added mass matrix of riser element mf Mass matrix of mud in riser element ms Structural mass matrix of riser element pw State of vessel WF model Rb,s,v Ice resistances due to bending and submersion... process plant and control plant models of a drilling vessel and drilling riser for the control design of the REAs; b propose an active positioning control using mooring line tensioning for PM systems to reduce the REAs in open sea; c carry out experimental tests to validate the proposed control strategy; d extend the control algorithm to limit the riser end bending stresses rather than the REAs; and e extend... the increasing use of moored systems in ice, Løset et al (1998) developed the model 29 Chapter 1 Introduction tests of a STL to study the feasibility and line tension in level ice, broken ice and pressure ridges Hansen and Løset (1999) simulated the behaviour of a vessel moored in broken ice and compared the simulation results with those obtained from ice tank tests Figure 1.5 Kulluk drilling vessel. .. Length of crushing area CA Coriolis and centripetal matrix of water added mass Ca Allowable stress factor of riser Cd Drag coefficient of riser Cf Design case factor of riser Cm Added mass coefficient of riser CRB Skew-symmetric Coriolis and centripetal matrix of vessel Cr Total structural damping matrix of riser D Draft of vessel Dint Internal diameter of riser pipe Dmo Vector of additional damping from... increasing use of marine risers connecting a surface vessel or a platform to the well-head (through a blowout preventer (BOP)) at the seabed as shown in Figure 1.1 Figure 1.1 Oil and gas work-over and drilling operations Marine risers are traditionally classified in two main groups, namely, production risers and drilling risers Production risers can be found in a broad range of fluidconveying applications . CONTROL STRATEGY FOR MARINE RISER OF POSITION- MOORED VESSEL IN OPEN AND LEVEL ICE- COVERED SEA NGUYEN HOANG DAT NATIONAL UNIVERSITY OF SINGAPORE 2011 CONTROL. marine riser during shallow water drilling operations by position- moored (PM) systems in open water and level ice- covered sea. Basically, an active positioning control using mooring line tensioning. Risers 20 1.2.2 Control of Marine Risers 25 1.2.3 Control of Riser End Angles 26 1.2.4 Station Keeping for Drilling Operations in Ice- covered Sea 27 1.3 Objectives and Scope 30 1.4 Outline